Fatigue Crack Paths 2003

- the cracks of group 1 specimens developed on clearly expressed oriented texture,

the cracks of group 2 specimens developed on planes inherited from primary performed

material texture which is seen from the shape of the origin quasi-brittle facets;

- when the specimen experiences under the cyclic loads with hold-time (dwell), its

fracture origin is a quasi-brittle facet and the formation of fatigue striations in the

fracture surface began in the best case at the boundary of origin zone that was

discovered for specimens numbers 1,3,4, 7-11;

- decrease in dwell-time of cyclic loads and, especially transition to a triangular

shape of cyclic loads possibly caused more active formation of fatigue striations in

fracture surfaces of the investigated specimens as it is observed for specimen numder 5;

crack growth period may be estimated by the fatigue striations spacing, taking in

consideration that the striations spacing value is behind the crack growth rate in the case

of quasi-brittle fracture of titanium alloys detected for the majority of the investigated

specimens and therefore the crack growth period estimated on the bases of the fatigue

striations spacing measurements must be corrected.

Estimations loading conditions and lifetime to failure for specimens

As a result of the performed fractographic investigations it was carried out the

systematization of the measured fatigue striations spacings for different specimens and

the crack length (depth). Assessment of the crack growth period was made by the

average value of the fatigue striations spacing for specimens for which the appropriate

change in striation spacing in the direction of crack growth wasn’t detected.

The revealed specific features enable to conclude that the specimen number 12

showed itself best of all, the crack propagation in this specimen was performed during

approximately 3200 cycles. It is worth to emphasize once again that this and other

subsequent assessments don’t take into account the possible discrepancy between the

fatigue striation spacing and the crack rate in the case of mainly brittle material

fracturing by mechanism of quasi-brittle faceted pattern relief formation. The results of

the VT3-1 titanium alloy investigation [2], wherein a close consistency between the

crack growth rate and the striation spacing value with the dominating faceted pattern

fatigue fracture surface (same as the quasi-brittle facet) was obtained, point to the

admissibility of such approach for estimating the fatigue crack growth period in

specimens.

The number 5 specimen fracture relief with dominating fatigue striations pattern was

taking place during fatigue crack growth about 2000 cycles. The number 2 specimen

fracture having the intermediate fracture surface relief continued during fatigue crack

growth approximately 1100 cycles.

So, assessment of fatigue crack growth period of other specimens was made by

approximately the same deduction that was introduced above. It was shown that

specimens numbers 1,3,4 and numbers 7-11 were subjected to cyclic loads with hold

time and have passed through 430-1000 cycles during fatigue crack growth. All results

of crack growth period calculation were used for estimation lifetime of tested

specimens.